In electrostatography, image charge patterns are formed on a support and are developed by treatment with electrostatically charged marking particles which are attracted to the charge patterns. These particles are called toner particles or, collectively, toner. The image charge pattern, also referred to as an electrostatic latent image, is formed on an insulative surface of an electrostatographic element by any of a variety of methods. For example, the electrostatic latent image may be formed electrophotographically as in office copiers and laser printers, by imagewise photo-induced dissipation of portions of an electrostatic field of uniform strength on the surface of a photoconductive layer formed on an electrically conductive substrate. Alternatively, the electrostatic latent image may be formed by direct electrical formation of an electrostatic field pattern on a dielectric surface.
One well-known type of electrostatographic developer comprises a dry mixture of pigmented, thermoplastic toner particles in powder form and carrier particles. Developers of this type are employed in cascade and magnetic brush development processes. The toner particles and carrier particles differ triboelectrically, such that during mixing to form the developer, the toner particles acquire a charge of one polarity and the carrier particles acquire a charge of the opposite polarity. The opposite charges cause the toner particles to cling to the carrier particles. During development, the electrostatic forces of the latent image, sometimes in combination with an additional applied field, attract the toner particles. The toner particles are pulled away from the carrier particles and become electrostatically attached, in image-wise pattern, to the latent image bearing surface. The resultant toner image can then be fixed, by application of heat or other known methods, or can be transferred to another surface and then fixed.
In such development methods the electrostatic attraction between the toner and carrier particles must be strong enough to hold the toner particles on the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are pulled away from the carrier particles and deposited on the latent image-bearing surface.
Carrier particles can comprise a metallic or non-metallic core material coated with a polymer. Carrier coating polymers that have heretofore been used include: silicone resin; acrylic polymers, such as, poly(methylmethacrylate); and vinyl polymers, such as polystyrene. One purpose of the coating can be to reduce the tendency of toner material or other developer additives to adhere permanently to carrier surfaces during developer use (often referred to as "scumming"). Another purpose has been to improve the charging characteristics of the carrier.
A problem encountered in the use of electrostatographic developers is "throw-off," which refers to toner powder thrown out of a developer mix as it is mechanically agitated within a development apparatus. Throw-off can cause unwanted background development in the image and contamination problems in the apparatus. Throw-off can increase as the developer is used, to such an extent that the developer must be replaced. A possible mechanism for this increase in throw-off is that the charging sites on the surface of the carrier particles become scummed. If the throw-off of the developer can be controlled so that it does not increase unduly over time, the developer will last longer and reduce the cost to the user.
Polymers that have been proposed heretofore as coatings for carriers include silicones, acrylic polymers, vinyl polymers and fluorocarbon polymers. Patents disclosing silicone polymer coatings for developer carriers or for other substrates include:
U.S. Pat. No. 5,068,301 which describes an organopolysiloxane coating composition for an electrophotographic carrier; U.S. Pat. No. 4,977,054 which discloses as the coating for a developer certain specific silicone resins; U.S. Pat. No. 5,200,287 which discloses a soft ferrite carrier core that is coated with a composition comprising a silicone resin and a carbon fluoride; Japanese patent publication 6/266169 which discloses a carrier for a negative developer which has a soft ferrite core (copper zinc ferrite) and a silicone coating with hydrophilic silica particles; Japanese patent publications JP 59232362, JP 02210365 and JP 01191155 disclosing soft ferrite carrier particles coated with a filled silicone resin; and U.S. Pat. No. 4,027,073 which discloses the use of silsesquioxanes as abrasion resistant coatings for substrates such as acrylic lenses.
There is a need for carriers for dry developers having an improved combination of properties, such as greater charge stability of the toner, faster charging rate, low toner throw off and improved R.H. stability for the toner charge. The cited references fail to disclose developer carriers having the coatings that characterize the carriers of the invention and that provide the desired improved combination of properties.